1. Field of the Invention
The invention is in the field of optical transmission systems and, more particularly but not exclusively, of those systems in which coherent detection is used. The invention relates to an optical device for controllably transforming the polarization of an optical signal.
2. Prior Art
In a coherent optical receiver, a received signal and the signal originating from a local oscillator can be ideally combined only if the polarization states of the signal are identical. The received signal normally enters via a non-polarization-maintaining light path, with the result that the polarization state of said signal is undefined on reception. One of the solutions to this problem is achieved by polarization control, in which case the polarization state of one of the two signals is altered in such a way that it corresponds to that of the other signal. Such a solution in which the polarization state of the local oscillator signal is altered is disclosed in reference [1]. The polarization control disclosed therein is carried out using a controllable polarization transformer which is driven via a feedback loop. In addition to some controllable polarization transformers constructed using discrete components, this reference describes an integrated optical version which is based on lithium niobate and which is disclosed in more detail in reference [2]. This known transformer comprises a controllable polarization mode converter sandwiched between two separately controllable phase shifters. Both the phase shifters and the polarization mode converter are based on electrooptical modification of the propagation of the TE component and the TM component in a monomodal channel-type waveguide. The actual conversion of a fraction of one polarization component into the other polarization component (TE .rarw.-.fwdarw.TM) takes place in the polarization mode converter. In an integrated optical version of a coherent optical receiver, not only optical, but opto-electronic and electrical signal processing has to take place on one chip. Since it is at present usual to chose a wavelength in the near infrared for the light signal, with the present prior art an integrated optical receiver provided with such a polarization transformer can only be produced using of semiconductor material such as indium phosphide (InP). A polarization mode converter which is based on InP and which makes use of the linear electrooptical effect in said material is disclosed in reference [3]. To produce a controllable polarization transformer based on the principle disclosed in reference [2], at least one phase shifter must be added thereto. The total length of the converter and the phase shifter is, however, fairly large, approximately 10 mm, compared with the available space on InP substrates which are usual at the present time. There is therefore a need to base a controllable polarization transformer on a modified principle which permits a shorter length for such a component when integrated on the basis of semiconductor materials such as InP.